The two countries have long delighted in mocking the other's perceived flaws.
PARIS -- In one case, the catalyst was Hurricane Katrina; in the other, a freak electrocution accident in a Paris suburb.
What followed -- drownings and dislocation in the United States, riots across France -- has forced each nation to confront problems of racism and poverty that are deeply entrenched but usually ignored.
The parallel soul-searching is taking place in two countries where politicians and pundits have long delighted in mocking the other's perceived hypocrisies and flaws.
"I'm not sure you can say that one country's system is better or worse than the other -- neither works very well," said Dominique Moisi of the French Institute of International Relations.
"Each government waits for the problems to occur in order to address them, and their first reaction is slow and inadequate."
The devastation wreaked by Katrina in New Orleans took a disproportionate toll on low-income blacks, with hundreds drowning and tens of thousands losing their homes to flooding in low-lying neighborhoods.
In France, the deaths of two Muslim youths hiding from police in an electricity substation triggered rioting nationwide in bleak, immigrant-filled suburban housing projects where joblessness and alienation are endemic.
"After Katrina, many French took an undisguised glee in poking the eyes of the Americans. ... They said this couldn't happen in France," said Steven Ekovich, a political science professor at the American University of Paris.
"Now, the French are just stunned, groping to make sense out of what's happening around them. It's very difficult to admit they have a race riot, but that's what it is."
Experts from both countries said the United States, with its painful history of slavery and segregation, has been more willing than France to acknowledge and address racial tensions.
"In France, issues of discrimination were not supposed to arise," said Francois Heisbourg, a leading French foreign policy analyst. "Officially, we're all equal. It's politically incorrect to say otherwise."
The principle of equality has such weight in France that authorities generally do not collect racial or ethnic demographic data and have shunned U.S.-style affirmative action programs.
More so than the United States, France has failed to propel significant numbers of its racial minorities to top-rung positions in government, business or the media.
"There may be racism in the United States, but nobody would say an African-American is not an American," said Charles Kupchan, director of Europe studies at the New York-based Council on Foreign Relations. "Muslims in France find themselves feeling like second-class citizens -- not really part of the French nation."
In the aftermath of Katrina, elements in the French media seized a new chance to expound on America's problems. Now the French unrest has given some Americans a chance to point at bad examples.
One of the major U.S. groups urging a crackdown on illegal immigration cited the French riots as evidence that President Bush should abandon plans to accommodate more foreigners under a guest worker program.
"France is being ripped apart by the unemployed and unassimilated offspring of their own failed guest worker programs of the 1970s and 1980s," said Dan Stein of the Federation for American Immigration Reform. "If we bring millions of guest workers to this country, they will never leave. ... We will face massive social problems and costs down the road."
Several commentators suggested that France, more so than the United States, was likely to be so chastened by the latest trauma that it would undertake concrete steps to fight poverty and discrimination. Others were skeptical.
"I'm not very optimistic that this will lead to powerful change in either country," said Thomas, the UCLA professor. "There are incredible pressures not to look at these questions."